1. Field of the Invention
The present invention relates to polyaddition products containing hydroxyl and uretdione groups, a process for their preparation, their use for the production of highly reactive polyurethane (PUR) powder coatings which are free from elimination products and which crosslink to give light-stable and weather-stable coating films of high gloss, and the powder coatings produced thereby.
2. Discussion of the Background
DE-C 30 30 572 presents a process for the preparation of polyaddition products which contain uretdione groups, and the products prepared accordingly. These are reaction products of the isocyanurate-free uretdione (UD) of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate, IPDI)—which can be prepared according to DE-C 30 30 513 or DE-C 37 39 549—with diols and, if desired, monoalcohols or monoamines. The reaction can be carried out in bulk or else in the presence of appropriate solvents. However, so far, in practice, this class of crosslinking agent has been produced in economically significant, saleable quantities only in a suitable solvent under mild conditions, at about 60° C., so as to avoid thermal ring cleavage during the synthesis. Preparation in bulk (i.e. in the absence of solvent) has not previously gone beyond the laboratory scale, since the viscosity reaches unmanageable levels during the reaction as a function of the molecular mass of the crosslinking agent. While DE-C 3030572 indicates that increasing the reaction temperature can operate as a means of controlling the reaction viscosity this measure is somewhat limited since higher temperatures can lead to detrimental effects on the reaction products.
This limit is also evident from the examples of DE-C 30 30 572 which use a product having a molecular weight of not more than 3,000 or at a uretdione/diol molar ratio of 5:4 for a uretdione grade having a free NCO content of 17 and a total NCO content of 37.8% by weight. If the free NCO content of the isophorone diisocyanate uretdione increases for a constant total NCO content (i.e. there is a simultaneous decrease in the degree of oligomerization and thus also in the molecular mass of the uretdione employed), then there is a corresponding decrease in the molecular mass of the uretdione group-containing polyaddition products. For the reasons given above, there seems little point in the preparation of uretdione group-containing polyaddition products of high molecular mass for use as crosslinking agents in the production of PUR powder coatings. This is also reflected in DE-C 30 30 539 and DE-C 30 30 588.
In DE-C 30 30 539 the molecular weights claimed are between 550 and 4,300, preferably between 1,500 and 2,000, and the uretdione/diol molar ratios are between 2:1 and 6:5, preferably between 3:2 and 5:4.
In DE-C 30 30 588 the claimed molecular weights are very similar, namely from 500 to 4,000, preferably between 1,450 and 2,800 at a uretdione/dial molar ratio comparable with that of DE-C 30 30 539.
Decisive disadvantages of the PUR powder coatings in accordance with the teachings of DE-C 30 30 539 and DE-C 30 30 588, which are free from elimination products, include the limited possibilities for combination with polymers containing hydroxyl groups. In DE-C 30 30 539, high functionalities of from ≧3.4 to ≦7, preferably from 3.7 to 4.5, were necessary. In order to achieve the high crosslinking density required for high-quality PUR powder coatings, it was necessary to compensate for the chain-terminating components of the crosslinking agent. In DE-C 30 30 588, it was necessary to take account of the free NCO content of the crosslinking agents in so far as the functionality of the hydroxyl group-containing polymers is to be adapted to the free NCO content in order to avoid gelling during extrusion, and therefore to suppress losses in the quality of the coatings. It was necessary to limit the OH functionality to from ≧2.2 to ≦3.5, preferably from 2.5 to 3.4. Complex investigation was and still is necessary in order to tailor the resin and curing agent to one another.
In DE-C 30 30 572, polyaddition products which contain uretdione groups and terminal OH groups are described in one example and are claimed. The scope corresponds to the polyaddition product specified above. However, the OH-terminal polyaddition products have so far acquired no importance in the PUR powder coating sector, since there was no economic value seen in comparing with the other crosslinking agents (see Example 5 of DE-C 30 30 572 in comparison with the remaining examples). The numbers speak for themselves. These and chemical reasons, namely the uncontrolled polyaddition of the free OH groups with NCO groups, which are additionally produced by uretdione cleavage during synthesis, have been considered as sufficient to attach no importance to this kind of crosslinking agent.